The present invention relates generally to an improved a magnetic recording medium of a thin ferromagnetic film formed by vacuum deposition and a method for manufacture thereof.
In the conventional process for manufacture of so-called coated type magnetic tapes, magnetic powder such as gamma-Fe.sub.2 O.sub.3 or CrO.sub.2 is mixed with a binder, and the mixture is applied over a substrate and cured. In this process, the binder cannot be eliminated because it is used to disperse the magnetic powder. As a result, the information recording density of the coated type magnetic tapes cannot be improved beyond a certain limit. In order to solve this problem, there has been invented and demonstrated a thin ferromagnetic type magnetic tape which may be manufactured without the use of a binder. The thin metal or ferromagnetic film may be prepared by several methods such as chemical plating, vacuum deposition, sputtering, ion plating and so on. Of these methods, vacuum deposition is most advantageous in that the evaporation rate is fast and this method is very simple. However, the thin ferromagnetic film type tape has the problem of satisfactorily increasing its coercive force. For instance, with the chemical plating method, studies and experiments of increasing the coercive force of a thin cobalt film by adding a suitable impurity such as phosphorus have been long conducted. With the ion plating method, a thin film is formed under a high pressure of the order of 10.sup.-2 to 10.sup.-3 torr so as to control the grain size, thereby improving the coercive force. The thin magnetic film type tape prepared by the chemical plating method is however not satisfactory in practice because the strength of joint between the thin metal film and the substrate is not sufficient and the film forming rate is too slow. The thin magnetic film prepared by the ion plating method is also not satisfactory because its thin film forming rate is too slow.
The film forming rate of the vacuum deposition method is faster than any other method, but the coercive force of thin metal film tapes fabricated by the vacuum deposition method is in general less than 100 Oe which is considerably less than 200 to 500 Oe of the conventional coated type magnetic tapes so that they are unsatisfactory in practice. The volume or bulk coercive force of Fe, Co or Ni is tens Oe at the highest. The coercive force of a thin film formed by the vacuum deposition of these metals is 100 Oe at the highest, because the ferromagnetic metal is evaporated in a nearly ideal vacuum (10.sup.-4 to 10.sup.-6 torr) in the form of atoms and is recrystalized on a substrate.
A method is known in which the beam of evaporating metal is made incident at an angle greater than 45.degree. on a substrate in order to increase the coercive force of a thin magnetic film. However, the evaporation or thin film forming rate is too low to be used in practice.